Geochemical influences on the geotechnical parameters of weathered Karamu Basalt are determined and the likely processes involved in basalt weathering are investigated. Geochemical and geotechnical parameters of basalt at different stages of weathering are evaluated; possible relationships between these two sets of parameters are determined and quantified; models are formulated based on statistical analysis; and an hypothesis is postulated to explain the weathering processes. At an abandoned basalt quarry, materials belonging to different weathering grades are observed and described. In situ geotechnical tests such as Schmidt rebound hardness, penetration resistance and vane shear and laboratory tests such as uniaxial compressive strength, point load, direct shear, Shore scleroscope hardness, California Bearing Ratio, water content, density, particle size distribution, Atterberg limits, permeability, X-ray fluorescence and diffraction, abrasion pH, electrical conductivity, petrography, and scanning electron microscopy are carried out. Using statistical methods, comparisons between geochemical and geotechnical parameters are determined, and their relationships shown in graphical form. Model equations, which depict these relationships quantitatively, are evaluated. Geochemical results show the following major element concentrations in fresh Karamu Basalt: SiO₂ 42.39 ± 1.66%; Al₂O₃ 12.04 ± 0.23%; MgO 12.53 ± 0.59%; CaO 10.82 ± 0.31%; FeO 10.34 ± 0.35%; Fe₂O₃ 3.09 ± 0.10%; TiO₂ 2.41 ± 0.11%; Na₂O 1.30 ± 0.35%; K₂O 0.70 ± 0.26%; MnO 0.18 ± 0.00%; and loss on ignition 3.68 ± 0.99 %. In this study, the loss on ignition was assumed to be the structural water (H₂O+) concentration. As weathering proceeds, there is a reduction in CaO, MgO and FeO and an increase of Al₂O₃, Fe₂O₃ and H₂O+. When the chemical concentrations are recalculated assuming a constant Al concentration, these trends remain the same for CaO, MgO, FeO, Fe₂O₃ and H₂O+. The following trace elements are also identified: V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Nd, Pb and Th. At the slight stage of weathering, an increase in concentration is shown by: V, Cr, Ni, Zn, Ga, Y, Zr, Nb, Ba, La, Ce, Nd, Pb and Th. Some trace elements follow the abundance patterns of the major elements containing similar charges and compatible ionic radii (Ba follows Ca while Ga follows Al). Principal component analysis indicates that CaO and MgO values serve as discriminators of fresh basalt. Petrographic studies show that fresh basalt is holocrystalline to hypocrystalline, fine grained, porphyritic with phenocrysts of olivine, titanaugite, plagioclase and rare titanomagnetite set in a groundmass composed of plagioclase laths, titanaugite, olivine, glass and titanomagnetite; approximately olivine 33%, titanaugite 28%, plagioclase 34%, glass 2% and titanomagnetite 2%. Apatite and chromite are accessory minerals. As weathering proceeds the mineral assemblage changes and finally the completely weathered basalt consists of only secondary minerals (clays, hematite and goethite). The weathering profile was divided into five grades according to the New Zealand Geomechanics Society Standards: fresh, slightly weathered, moderately weathered, highly weathered and completely weathered. Only some geotechnical results could be obtained for all grades of weathering due to the varying nature of the testing materials. They are California Bearing Ratio (CBR), density, porosity and water content. Fresh rock is strong (uniaxial compressive strength ∼ 262 MPa) with near-vertical columnar jointing. Along joint planes, discolouration occurs at slightly weathered stage. From fresh to slightly weathered stage, there is a dramatic drop in strength parameters (average point load strength index 5.59 for fresh rock, 0.41 for slightly weathered basalt; CBR drops from 100 % to 45 %), and horizontal joints develop. From fresh to completely weathered CBR drops from 100 % to 1 %; density drops from 2902 kg m⁻³ to1502 kg m⁻³; and porosity increases from 0.18% to 27.19 %. From slight to completely weathered stage cohesion and water content increase (cohesion 0 to 7 kPa; water content 12 % to 59 %). From the moderately weathered stage, corestones are reduced in size and a clayey matrix develops. The material shows plastic behaviour, but Atterberg limits show no consistent trends with weathering grade. Based on chemical analyses, an easily calculable “new weathering index” (NWI): NWI = (MgO+CaO+FeO)/(Al₂O₃+Fe₂O₃+H₂O+) which may be used to define the degree of weathering of Karamu Basalt, is proposed. It correlates well with compressive strength, water content, density, porosity, and California Bearing Ratio but does not correlate well with shear strength, Atterberg limits, cohesion or angle of internal friction. Abrasion pH value is indicative of the degree of weathering. It is higher in fresh rock (8.5) and gradually decreases to 4 due to weathering. Abrasion pH has good positive correlations with compressive strength, density, and California Bearing Ratio and negative correlations with water content, cohesion, and porosity. By obtaining the abrasion pH of Karamu Basalt material, its degree of weathering can be determined. Several chemical predictors of geotechnical parameters are formulated. Predictor “x”, given by: x = 225 pH + 283 TiO₂ + 7.4 Sr - 80 Fe₂O₃ provides the best prediction of geotechnical properties of Karamu Basalt. Its suitability for other lithologies could not be evaluated due to unavailability of data regarding trace element content and abrasion pH value. Modified versions with fewer components were tested against other lithologies, with some success for specific parameters. Obtaining trace element analysis and abrasion pH values is thus crucial for predicting geotechnical parameters from geochemical data. A causal relationship between the loss of cations Mg and Ca and initial loss of strength is hypothesised. The early stage of weathering is diffusion controlled (with some hydrolysis) whereby cations are lost from the constituent primary minerals and are replaced by H⁺ and possibly Al³⁺. This process affects the lattice structure due to radius to charge ratio imbalances, thus causing a marked loss of strength, leading to microfractures and later macrofracturing following stress release. Later weathering is controlled by hydrolysis, dissolution, Redox and leaching, leading to the development of clay minerals. Based on the results of this study, it is suggested that the basalt weathering profile be divided into only 3 categories as follows: fresh rock, weathered rock and saprolitic soil. Fresh rock should remain as it is; slightly weathered, together with moderately weathered material should be called weathered rock; highly and completely weathered material should be included in the saprolitic soil. This is a simpler classification, and better reflects the geotechnical behaviour of the materials. Further investigation may indicate the suitability of this simpler classification for other lithologies.